1. Field of the Invention
This invention relates to silver halide photographic materials which may be handled under normal lighting conditions without fogging and which are capable of recording images of high intensity light or other actinic radiation.
2. Prior Art
The majority of photographic recording materials require specially filtered and subdued light in order to be handled safely without fogging. This applies particularly to silver halide photographic recording materials which are, in general, more light sensitive than non-silver materials. Silver halide recording materials which have not been spectrally sensitized by the addition of sensitizing dye are normally only highly sensitive to ultraviolet and blue light. Such materials may be safely handled in subdued yellow or red light. It is very desirable to produce a silver halide photographic recording film which may be safely handled in bright white light. The benefits of this include ease of working and inspection of the material during exposure and processing, and generally more pleasant working conditions for the operators.
The term "white light" used herein is defined as the emission of a typical commercially available "white" or "warm white" fluorescent tube which has relatively little emission shorter than 400 nm wavelength, or longer than 700 nm. Various techniques have been employed in order to achieve some degree of white light handleability.
Silver halide materials which are intended for exposure to wavelengths outside the above range; e.g., direct X-ray recording materials or ultraviolet recording materials, may be afforded some degree of "white light handleability" by overcoating the radiation sensitive layer with a layer containing a filter dye. Typically, the dye will absorb visible light, particularly blue light, and prevent exposure of the radiation sensitive layer by this light. The filter dye is chosen such that it does not greatly interfere with intentional exposure of the material to U.V. or X-rays. Examples of constructions incorporating such filter layers are disclosed in U.S. Pat. Nos. 4,140,531 and 4,232,116 which employ blue absorbing dyes which are bleached during processing.
The improvement in white light handling safety which may be achieved by the use of filter layers is not normally very great unless very high dye loadings are used. Such high dye loading tends to cause problems of residual dye stain.
An alternative method for achieving handleability in white light is to add a substance to the radiation sensitive layer which causes desensitization to radiation of all wavelengths. Materials desensitized in this way must then be exposed to a very powerful light source, much brighter than the room light in which they can be handled. Silver halide materials desensitized by the addition of rhodium salts are disclosed, for example, in Japanese Patent Specification No. J5 6125-734. The white light handleability of materials desensitized in this way is not very great unless impractically high levels of desensitization are effected.
Materials for direct X-ray recording may be rendered handleable by the addition, to the silver halide layer, of compounds which desensitize the silver halide to visible (white) light, but not to X-rays. U.S. Pat. Nos. 3,671,254; 3,237,008 and 3,658,547 disclose examples of such components.
The most effective known method of imparting safe handling characteristics in white light is by the incorporation into the silver halide emulsion of compounds which induce a high degree of low intensity reciprocity failure (L.I.R.F.). This effect is described in The Theory of the Photographic Process, Ed. T. H. James, 4th Edition 1977, Macmillan, page 133. L.I.R.F. is an effect whereby a silver halide emulsion may become insensitive to low intensity exposures even at extended exposure times. It is possible by the use of specific additives to accentuate this effect to such an extent that a silver halide emulsion cannot be fogged by low intensity light even after many hours exposure. By suitable choice of silver halide emulsion and additive it is possible to arrange that the normal level of white room-lighting is below the lower limit of intensity by which the emulsion can be exposed. Emulsions modified in this way may still be exposed by exposing radiation of higher intensity than normal room light.
Compounds which promote this effect will hereafter be referred to as "L.I.R.F. inducers". The application of L.I.R.F. inducers to white light handleable photographic film has been described in Professional Printer, 1981, Vol. 25(2), page 9.
There are many classes or organic compounds which act as desensitizers towards silver halide emulsions; see, for example, P. Glafkides, Photographic Chemistry, Chapter XLII, Fountain Press, 1960. Classes of organic compound which are known to form derivatives which may act as desensitizers include: phenazines, tetrazines, quinoxalines, aromatic nitro compounds, azo dyes, anthraquinones, diphenyl methanes, triphenylmethanes, cyanines, nitrated cyanines, styryl derivatives, styryl vinyl derivatives, benzopyrilium derivatives furyl vinyl quinolines, methine-anthraquinone derivatives, anils, aza cyanines, triazoles, dipyridine derivatives and phenazine N-oxides. Many desensitizers for silver halide emulsions also act to some extent as L.I.R.F. inducers. (J. Opt. Soc. Amer., 1955, 45, 15; and Phot. Sci. Eng., 1967, 11, 82). However, not all compounds which are known to be desensitizers are equally efficient in this respect. A compound could not be predicted to be useful as a L.I.R.F. inducer for white light handleable film simply from the knowledge that it is a desensitizer. Furthermore, not all compounds which are L.I.R.F. inducers are equally useful to white light handling film, since many such compounds cause the latent image present on imaged film to fade when the film is exposed to room light even though the intensity of the room light may be insufficient to cause fogging.
It is an object of the present invention to provide white light handleable photographic elements incorporating L.I.R.F. inducers.